Tectonic Summary

The Bonin Islands earthquake of December 21, 2010, occurred as the result of normal-faulting within the Pacific plate, in the region of the boundary between the Pacific plate and the Philippine Sea plate. In the epicentral region of the earthquake, the Pacific plate moves west relative to the Philippine Sea plate with a velocity of about 4 cm/yr. The Pacific plate subducts beneath the Philippine plate at the Izu trench and is seismically active west of the epicenter of the December 21 earthquake to a depth of about 560 km. The stresses that generated the December 21 earthquake result from the bending of the Pacific plate as it subducts beneath the Philippine Sea plate.

The Izu-Bonin and Mariana arcs of the Pacific/Philippine Sea plate-boundary region experience frequent moderate and strong earthquakes. In the past quarter century, the thousand kilometer section of the plate boundary centered on the epicenter of the December 21 earthquake has produced 27 earthquakes of magnitude 6 or greater, with 3 having magnitudes larger than 7.0. Roughly half of the moderate and large shocks in this region occur as intermediate-depth earthquakes (70 300 km below the earths surface) or deep-focus earthquakes (greater than 300 km below the earths surface). The December 21 earthquake, however, occurred within several tens of kilometers of the earths surface and would be classified as a shallow-focus earthquake

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